inferior mirage

   The term inferior mirage is indebted to the French verb se mirer, which means to reflect, or to be reflected. The adjective inferior refers to the phenomenon's position relevant to the horizon or a distant object. The term inferior mirage is used as a generic term for a group of "physical illusions typically depicting distant pools of water or oil or blue sky over a hot surface such as a road, a runway, or a desert, which have a relative position beneath a perceived distant object or the horizon. If the light is reflected not by the sky but by a mountain or other distant object, then that object is reflected in the miraged image. Inferior mirages tend to be classified in accordance with the nature of the surface above which they appear. Two examples of inferior mirages are the desert mirage and the " highway mirage. The mediation of inferior mirages is attributed to differences in the refractive index of the atmosphere, which are in turn attributed to layers of cold air overlying layers of hot air, with differences in temperature between the adjacent layers of air of 10° C or more per metre. Uniform temperature gradients tend to produce undistorted images. Increasing temperature gradients, such as those of 10°C/m near the ground, and 20°C/m higher up, tend to yield inverted images. Temperature gradients of a higher complexity may yield even more complex distortions. These may be vibrating, vertically extended (i.e. 'towering'), flattened (i.e. 'stooping'), etc. The term inferior mirage is used in opposition to the terms " superior mirage, "double mirage, and "lateral mirage.

   References

   Fraser, A.B. (1979). Simple solution for obtaining a temperature profile from the inferior mirage. Applied Optics, 18, 1724-1731.

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.